1. Field of the Invention
This invention relates to a rotary screen printing machine. More particularly, it relates to a rotary screen for use therewith and especially relates to a device for uniformly supporting the end rings of the rotary screen against axially directed pressure of the longitudinal tensioning means therefor.
2. Review of the Prior Art
In recent years increasing interest has been focused upon rotary screen printing machines for use in textile printing. Due to the advances in the automation of textile production, there has been a growing need for a system to apply patterns to textiles both economically and at a high rate of production. Until recent years there were two main systems of applying patterns to textiles by printing, namely: (1) flat bed printing and (2) roller printing. While flat bed printing was relatively inexpensive, it was an inherently intermittent process which offered a rate of production much too slow to satisfy the textile industry. The rate of production of roller printing, however, represented a multiple increase over flat bed printing, which it essentially superseded. However, roller printing also represented higher costs and capitalization and thus a demand for longer runs. Consequently, with advances towards automation of textile production, there has been a growing need for a system of applying patterns to textiles which will not require the high capitalization, the vast runs of the roller printing machine, and the large amount of down time, all common to roller printing, and which will yet be appreciably faster than flat bed printing.
A process which apparently offers a solution to the foregoing problems and which is being increasingly accepted by industry is rotary screen printing. The apparatus utilized in such system comprises a perforated cylindrical or rotary screen which is utilized to apply colored designs to the textiles. The color applied is in the form of an emulsion or paste which is forced from the interior of the cylindrical or rotary screen through the perforations of the screen and onto the textile workpiece in a pattern according to the engraving of said cylindrical or rotary screen.
Generally, the screen per se is relatively thin, e.g., about 0.08 to 0.20 mm (about 0.003 to 0.008 inches). In operation, the colored emulsion or paste is passed from a reservoir inside the rotary screen by means of a squeegee which controls emulsion application, but it is the contact of the outer area of the screen on the textile substrate which determines the amount of pressure applied to the actual print.
The advantages of rotary screen printing vis-a-vis flat bed and roller printing are numerous. For example, in flat bed printing machines, the overlap between screens on a flat machine can easily cause smudges and crush the still-wet pattern of the adjacent but overlapped newly printed surface. Such defect will be evidenced as "cross-bars". Consequently, it is important that printing pastes be compounded so that they are absorbed by the fabric immediately on application; otherwise, crushing is likely to result.
In addition, because of the relatively large area covered by a single screen brought into contact with the textile, there is a danger of such textile being lifted with the screen and torn away from the adhesive on the printing blanket of the flat bed. Such a problem is often encountered with lightweight synthetics. Consequently, a detrimental compromise must be made on the gauze of the screen in relation to the substrate: to use non-sticky printing pastes; and to use low-viscosity, rapidly absorbed pastes. Such a compromise results in loss of print sharpness, loss of brilliance, and loss of dyestuff development.
The advantages of rotary screen printing over roller printing reside in the fact that: (1) it is inevitable that color is taken from one roller to the next; (2) because of the high pressures which must be utilized in roller printing, the crushing of newly printed surfaces is virtually impossible to prevent; and (3) a much faster turn-around time is provided, i.e., time to change patterns on the printing machine. This results in the ancillary benefits of:
a. Reduced labor costs, PA1 b. Overall productivity exceeding roller printing on short and medium length runs, and PA1 c. Economical short runs.
The advantages of rotary screen printing compared with flat bed printing and roller printing are not only technological but are also evidenced by economic gains such as (1) the aforementioned improved productivity; (2) lower capital costs are required for the rotary screen due to low material cost, e.g., no copper inventory is needed; (3) the cost of screen engraving per se is about equal for rotary and flat screens and substantially less than that of engraving a copper roller; and (4) color consumption is minimal but without sacrifice of color yields which are relatively high.
While rotary printing evidences many advantages, certain problems are existent which are detrimental to the process. One such problem area is found with a rotary screen per se and the method of its fabrication. Heretofore, some of the earliest rotary screens were fabricated from phosphor bronze mesh, made first as a flat screen and then soldered at the repeat join to form a circular screen. The disadvantages of such screens, however, was in the soldered join and in the metal from which the screens were fabricated. Large open patterns and continual stripes could not be printed and the screens did not have a long life. Subsequently, woven circular bronze or stainless steel sleeves were also utilized, but these also presented difficulties in the usage and color application. An improvement was represented by the Galvano-plastic method which involved the electrolytic deposition of metal onto a matrix of steel which had previously been impressed with a specific number of dots corresponding to certain mesh sizes. The dots on the matrix were filled with a dielectric, and when the resulting matrix was entered into a plating bath a sheet was produced having perforations corresponding in size and number to the non-treated dots. Such rotary screens, however, often damaged easily and wore out quickly due to the corrosive action of the colored printing emulsions and pastes.
Another improvement was represented by the use of nickel. Nickel was eventually the metal chosen for deposition on the mesh since it is relatively inert to the chemicals encountered in the emulsions utilized. Another rotary screen developed was an all-metal screen fabricated by photographing the desired design image onto the circular matrix and depositing nickel thereon to a specifically defined thickness, the result being an all-metal sleeve containing the design and the necessary perforations. Both of the latter types of rotary screens, however, are also easily damaged, e.g., by corrosion, especially by acid dyes, or dented and the imperfections resulting therefrom show up in the resultant printing and are particularly unsatisfactory and costly, especially when such damage occurs during the peak of a long printing run. Another disadvantage inherent in the known rotary screens is the cost of fabricating the rotary screen per se because of the cost of preparation or engraving the screen, as well as the cost of the metal, e.g., nickel, utilized therein. There was, therefore, a bona fide need for an improved rotary screen fabricated from a material which overcomes the problems of damage or denting but which can be fabricated at relatively low cost.
Accordingly, co-pending Ser. No. 528,736, filed Nov. 4, 1974, relates to a precision plastic cylinder for use in the manufacture of an improved rotary printing screen for rotary screen printing. It also relates to a process for efficiently fabricating a substantially chemically inert, dimensionally stable precision plastic rotary screen from film, which rotary screen is advantageously utilizable in rotary screen printing processes and overcomes most, if not all, of the prior art deficiencies noted above. The plastic cylinders suitable for use as blanks in the preparation of rotary printing screens for use in rotary screen printing which are provided therein have critical dimensions and characteristics. The rotary cylinders are from about 48 inches to about 200 inches along the longitudinal length with a circumference of from about 18 to 78 inches. They are formed from a single sheet or a laminate sheet of essentially chemically inert, dimensionally stable film, such sheet or laminate being from about 0.002 to about 0.025 inch in thickness.
In fabrication, the sheet edges are preferably abutted so as to provide a join which precludes imperfections in the printed product when employed in rotary screen printing. A skewed join is advantageously employed to obtain a precise circumference of the cylinder. The joining of the sheet edges may be also done in the conventional manner, but ultrasonic sealing is preferred.
A rotary screen suitable for use in rotary screen printing operations is also contemplated therein and is produced by subjecting said plastic cylinder blank to a controlled energized beam, e.g., laser beam, of sufficient intensity to decompose the plastic material and form minute holes which extend through the cylinder.
While such plastic cylinders represent a technical advance of substantial merit, some problems are incurred when adapting them to practical use on rotary printing machines. In order to describe said problems, some attention must be given to the rotary machines per se with which the cylinders are to be utilized as well as to the techniques employed in their operation. Printing machines of the aforementioned type are usually equipped with a number of printing stations, each of which incorporates a respective rotary screen which rotates in synchronism with the backing cloth or printer's blanket which is situated beneath the material or fabric to be printed. Furthermore, beneath each rotary screen and below the printer's blanket there is arranged a counterpressure roller which supports the printer's blanket. Occasionally, each printing station employing a rotary screen is preceded or followed sequentially by one or more "crush" rollers. In all instances it is of extreme importance for the proper functioning of the rotary screen printing machine that the printing screens and the various rollers can be easily exchanged and/or adjusted at any time required. Additionally, the centering of the rotary printing screen must be as accurate as possible.
The rotary screen at each printing station is herein defined as comprising a screen cylinder, a pair of end-rings, and a pair of support members. As mentioned, the screen cylinders are formed of very thin material having a thickness of only several thousandths of an inch. At the edges of these screen cylinders there are positioned endpieces or endrings by means of which the rotary screen unit can be centered and fastened to the printing machine.
In order to insure a frictionless working of the rotary screens in the printing machines, the thin-walled screen cylinder must be suitably attached in the machine and be tensioned in the axial or longitudinal direction. For this purpose, various tensioning devices are utilized which are arranged either inside the hollow screen cylinder or which are worked on the outside from the machine frame. U.S. Pat. No. 3,556,004 relates to one such device on rotary printing machines for exchangeably receiving and longitudinally tensioning rotary screens by connection of their endrings with rotary heads which are coordinated to the machine frame on both sides.
It is obvious that the longitudinal tension applied to the endrings and through the axis of the cylinder can be a cause of major concern. For example, even though the cylinder is held to the endring by adhesive means, the adhesive bond can be loosened at least in part, which can cause an off-centering or "cocking" of the endring relative to the cylinder. Such loosening or "cocking" can cause damage to the cylinder, e.g., by cracking, and to the workpiece textile as well. If the cylinder utilized is seamed in any fashion, such seam is especially vulnerable to the loosening or "cocking" of the endring. The loosening can also cause the entire removal of the endring with the resultant breakdown of at least that particular printing station.